1. Field of the Invention
The present invention relates to an apparatus for determining the bending characteristics of an elongated member and more particularly to a method and apparatus for testing the bending characteristics of a curved surgical needle subjected to linear or rotational forces or any combination thereof. In particular, such bending moment deformation facilitates obtaining the bending characteristics of the needle, including bending yield moment, ultimate bending moment, and bending stiffness.
2. Description of the Related Art
While using a curved surgical needle a surgeon must know approximately how much the needle can bend or deform and still retain its original shape. It is well known that at some point the deformation may become permanent. As a force is exerted on an area of a needle, the needle begins to bend and undergo deformation. As with many metals, initially the deformation is elastic, that is, the needle will return to its original shape when the force is removed and no hysteresis will be evident. In the needle art this elastic range is often regarded as a measure of stiffness of the needle.
A needle may be tested for this elastic range by loading and unloading the needle at a particular point and recording the differences in the load data. During elastic deformation the results will be plotted as a straight line on a graph of load vs. angular deformation. At some point the deformation may become permanent, i.e., inelastic. The level of loading which causes permanent deformation is called the yield load, which has been empirically determined to occur at a point where the straight line plot of load vs. strain crosses a line 2.degree. parallel to, and offset from the straight portion of the curve.
There are various devices available to test the bending strengths and yield loads of surgical needles. One known device, the Tinius-Olson Stiffness Tester, is designed to test the bending strength of straight needles by forcing a tip of the needle against a weighted plate and observing the deflection of the plate. Other known devices used to measure the bending strength of curved needles typically employ a projection, such as a peg or knife edge, to intercept a rotating needle somewhere along its length.
One known device is described in commonly assigned U.S. Pat. No. 5,022,273 to Evans, the disclosure of which is incorporated herein by reference. The Evans Patent relates to a needle tester which includes a clamp mounted on a rotary table driven by a stepper motor. Horizontal and vertical manual vernier positioning means are provided to aid in setting an initial position. A knife edge is mounted on a load cell to intercept a rotating needle while undergoing test loads while the load vs. angular deformation of the needle is being recorded. The Evans device is structured to maintain the contact edge equidistant from the center of rotation of the needle during the rotation. This method assumes that the load forces are purely vertical. However, any knife edge, no matter how sharp, has a finite radius of curvature causing a horizontal force component to be introduced and therefore the reaction of a knife edge is a radial force component rather than a true vertical component.
While the recent devices measure the bending strength of a curved surgical needle loaded along its length they provide little information about the behavior of such curved surgical needle when loaded at or near the tip of the needle. For example, the greatest amount of the deformation of the needles will normally occur when loaded at or near the tip. Since the loads on the tip of a needle are generally substantial during penetration of tissue, the bend strength of a needle so loaded is of great interest to surgeons. Furthermore, during such use curved surgical needles are subjected to linear as well as rotational forces and it would therefore be advantageous to test a needle under these compound force conditions.